The present invention relates to a cell provided with a gaseous diffusion electrode and a method of charging and discharging the same. Secondary cells are widely used in a variety of electric apparatuses and as mobile power sources. Known representative examples of such secondary cells include lead batteries and alkaline batteries.
In such alkaline batteries, a nickel electrode is predominantly used as the positive electrode, but an air electrode or a silver oxide electrode is also used instead in some such batteries. Cadmium, zinc, hydrogen, iron, etc. are usable in negative electrodes, but the use of a cadmium electrode is mainstream. Further, a nickel-hydrogen battery wherein use is made of a hydrogen-absorbing alloy in order to attain a high energy density has been put into practical use.
Meanwhile, investigations have heretofore been made of re-chargeable cells provided with a gaseous diffusion electrode as the positive electrode thereof in order to materialize a high energy density when they are used as mobile power sources, power sources for storage of electric power, etc.
The cells of this type are provided with a chargeable auxiliary electrode in order to prevent the gaseous diffusion electrode from deteriorating because of oxidation thereof during the course of charging. Usually usable examples of the chargeable auxiliary electrode include a nickel screen, and expanded metals from the viewpoint of electrolytic solution resistance, alkali resistance, etc. On the other hand, zinc or the like is usually used as the negative electrodes of the above-mentioned cells, but involves a disadvantage that the life span thereof is short. On the other hand, iron having a comparatively long life span is liable not only to generate hydrogen but also to consume an electrolytic solution because it is poor in charging efficiency. Further, when the side of the negative electrode is involved in a rate-determining step, the capacity of the cell is lowered through charge-discharge thereof. By contrast, since the hydrogen-absorbing alloy is high in charging efficiency, long in life span and little causative of pollution, a cell wherein the hydrogen-absorbing alloy is used as the negative electrode thereof was developed as well. In this kind of cell, wherein oxygen or air is used as the active material of the positive electrode thereof, a high energy density can be accomplished because the active material of the pole exists predominantly inside the cell.
Since air or oxygen on the outside of the cell is inherently used in the above-mentioned cell, however, gaseous diffusion electrodes as the positive electrodes thereof must be installed on both sidewalls of the battery container thereof, with the result that many positive electrodes and negative electrodes cannot be installed in a piled-up state on the inside of the cell unlike other kinds of cells. More specifically, the maximum number of positive electrodes in the cell is 2. Accordingly, the number of negative electrodes as counter electrodes is not required to exceed 2 because no benefit can be secured thereby. When discharging is rapidly carried out, therefore, the current density of the electrodes is increased to notably lower the voltage therebetween. Further, since no active material itself exists in the positive electrodes, oxygen is generated from the beginning of charging in the case of charging to heighten the charging potential, with the result that a high energy power efficiency (Wh efficiency) during the course of either charging or discharging cannot be expected even if the hydrogen-absorbing alloy is used in the negative electrodes.
Accordingly, a primary object of the present invention is to provide a cell capable, of course, of low output discharge like the conventional cells provided with a gaseous diffusion electrode, excellent in high output discharge properties at a high discharge voltage, and capable of exhibiting a high energy efficiency; and a method of charging and discharging the same.